The utility of live
attenuated vaccines for controlling HIV epidemics is being debated. Live attenuated
HIV vaccines (LAHVs) could be extremely effective in protecting against
infection with wild-type strains, but may not be completely safe as the attenuated strain could cause
AIDS in some vaccinated individuals. We present a theoretical framework for evaluating the consequences of the tradeoff between
vaccine efficacy (in terms of preventing new
infections with wild-type strains) and safety (in terms of
vaccine-induced
AIDS deaths). We use our framework to predict, for Zimbabwe and Thailand, the epidemiological impact of 1,000 different (specified by efficacy and safety characteristics) LAHVs. We predict that paradoxically: (i) in Zimbabwe (where transmission is high) LAHVs would significantly decrease the
AIDS death rate, but (ii) in Thailand (where transmission is low) exactly the same
vaccines (in terms of efficacy and safety characteristics) would increase the
AIDS death rate. Our results imply that a threshold transmission rate exists that determines whether any given LAHV has a beneficial or a detrimental impact. We also determine the
vaccine perversity point, which is defined in terms of the fraction of vaccinated individuals who progress to
AIDS as a result of the
vaccine strain. Vaccination with any LAHV that causes more than 5% of vaccinated individuals to progress to
AIDS in 25 years would, even 50 years later, lead to perversity (i.e., increase the annual
AIDS death rate) in Thailand; these same
vaccines would lead to decreases in the annual
AIDS death rate in Zimbabwe.